It is known from DE-OS 38 18 697 to use for chemical and physical pressure reactions conventional microwave ovens and pressure resistant container inserts which serve to receive the samples and are inserted singly or several at a time in the heating chamber defined by the housing of the respective microwave oven. The microwaves emitted by the microwave generator of the microwave oven into its heating chamber when in operation penetrate the microwave-permeable plastics container wall and heat up the sample in the container, with a corresponding increase in pressure. Since the container inserts consist of plastics material they may be used without suffering damage only up to pressures of up to 200 bar. Higher pressures are however desired, for example for extractions, to increase the boiling point of the solvent so as to perform the extraction at correspondingly higher temperatures and thus more quickly, i.e. more economically. While pressure resistant container inserts of metallic material withstand higher pressures, they are not suitable for use in microwave ovens because of their property of reflecting microwaves.